﻿J. 
  AV. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  241 
  

  

  gradually 
  removed 
  : 
  and, 
  under 
  favourable 
  circumstances, 
  the 
  original 
  

   crystals 
  are 
  sometimes 
  thus 
  again 
  exposed, 
  exhibiting 
  all 
  their 
  pris- 
  

   tine 
  lustre 
  and 
  beauty. 
  Among 
  the 
  minerals 
  thus 
  occurring 
  many 
  

   beautiful 
  varieties 
  of 
  the 
  epidote 
  and 
  garnet 
  groups 
  are 
  especially 
  

   conspicuous. 
  Some 
  of 
  these 
  minerals 
  appear 
  to 
  have 
  been 
  formed 
  

   in 
  the 
  fissures 
  of 
  masses 
  of 
  rock 
  which 
  were 
  ejected 
  from 
  the 
  

   volcanic 
  vents 
  ; 
  others 
  may 
  have 
  originated 
  from 
  the 
  action 
  of 
  

   various 
  gases 
  upon 
  the 
  materials 
  of 
  the 
  lava 
  during 
  the 
  " 
  solfatara 
  

   stage 
  " 
  of 
  the 
  volcano's 
  history. 
  

  

  d. 
  Volcanic 
  Breccias. 
  — 
  -The 
  rocks 
  of 
  the 
  last-mentioned 
  class 
  pass 
  

   by 
  insensible 
  gradations 
  into 
  those 
  which 
  it 
  is 
  now 
  our 
  purpose 
  to 
  

   describe. 
  Scattered 
  through 
  the 
  masses 
  of 
  the 
  agglomerates 
  of 
  

   scoriae 
  and 
  ashes, 
  we 
  find 
  blocks 
  of 
  stone 
  of 
  the 
  most 
  various 
  kinds, 
  

   which 
  are 
  evidently 
  of 
  foreign 
  origin 
  ; 
  and 
  in 
  some 
  cases 
  these 
  

   become 
  so 
  numerous 
  that 
  the 
  rock 
  may 
  be 
  described 
  as 
  a 
  breccia 
  

   composed 
  of 
  fragments 
  of 
  foreign 
  rocks 
  imbedded 
  in 
  a 
  matrix 
  of 
  

   volcanic 
  ashes, 
  sand, 
  and 
  lapilli. 
  

  

  That 
  these 
  angular 
  blocks, 
  which 
  vary 
  in 
  size 
  from 
  masses 
  several 
  

   tons 
  in 
  weight 
  down 
  to 
  the 
  smallest 
  fragments 
  visible 
  to 
  the 
  naked 
  

   eye, 
  were 
  actually 
  ejected 
  from 
  volcanic 
  vents, 
  we 
  should 
  be 
  justified 
  

   in 
  concluding, 
  not 
  only 
  from 
  the 
  analogy 
  of 
  existing 
  volcanoes, 
  but 
  

   from 
  the 
  confused 
  and 
  irregular 
  manner 
  in 
  which 
  they 
  lie 
  in 
  the 
  

   masses 
  of 
  evidently 
  erupted 
  volcanic 
  materials*. 
  Fortunately, 
  how- 
  

   ever, 
  we 
  can 
  point 
  to 
  facts 
  which 
  are 
  altogether 
  conclusive 
  as 
  to 
  the 
  

   origin 
  of 
  these 
  blocks. 
  They 
  are 
  found 
  to 
  belong, 
  in 
  every 
  case, 
  to 
  

   the 
  particular 
  rocks 
  through 
  which 
  the 
  volcanic 
  vent 
  which 
  they 
  sur- 
  

   round 
  has 
  been 
  opened. 
  Thus 
  in 
  the 
  island 
  of 
  Rum 
  (where 
  these 
  

   volcanic 
  breccias 
  are 
  admirably 
  displayed, 
  as 
  we 
  shall 
  see 
  hereafter), 
  

   which 
  forms 
  the 
  central 
  part 
  of 
  an 
  old 
  volcano 
  that 
  has 
  burst 
  

   through 
  rocks 
  of 
  Cambrian 
  sandstone, 
  the 
  blocks 
  in 
  question 
  consist 
  

   exclusively 
  of 
  materials 
  derived 
  from 
  that 
  formation 
  ; 
  in 
  Mull 
  a 
  

   great 
  volcano 
  has 
  originated 
  in 
  the 
  midst 
  of 
  various 
  Palaeozoic 
  and 
  

  

  * 
  The 
  frequency 
  with 
  which 
  blocks 
  torn 
  from 
  the 
  rocks 
  underlying 
  a 
  volcano 
  

   are 
  thrown 
  from 
  its 
  vents 
  during 
  eruptions 
  is 
  a 
  fact 
  familiar 
  to 
  all 
  students 
  of 
  

   volcanic 
  geology. 
  Alike 
  in 
  the 
  old 
  agglomerates 
  of 
  Somma 
  and 
  among 
  the 
  

   modern 
  ejections 
  of 
  Vesuvius, 
  there 
  abound 
  fragments 
  of 
  those 
  stratified 
  rocks 
  

   in 
  the 
  midst 
  of 
  which 
  the 
  great 
  fissures 
  to 
  which 
  the 
  Neapolitan 
  volcanoes 
  owe 
  

   their 
  origin 
  have 
  been 
  opened. 
  The 
  so-called 
  " 
  lava 
  " 
  ornaments 
  of 
  Naples 
  are 
  

   made 
  from 
  the 
  limestones, 
  often 
  more 
  or 
  less 
  dolomitized, 
  of 
  these 
  ejected 
  blocks 
  ; 
  

   while 
  from 
  the 
  cracks 
  and 
  cavities 
  of 
  the 
  same 
  are 
  derived 
  the 
  greater 
  portion 
  of 
  

   those 
  beautifully 
  crystallized 
  specimens 
  which 
  have 
  made 
  Somma 
  and 
  Vesuvius 
  

   so 
  preeminently 
  famous 
  among 
  mineralogists. 
  The 
  blocks 
  thrown 
  out 
  of 
  the 
  

   great 
  Neapolitan 
  volcano 
  were, 
  for 
  the 
  most 
  part, 
  evidently 
  derived 
  from 
  the 
  

   Subapennine 
  (Tertiary) 
  strata; 
  but 
  some 
  from 
  the 
  Apennine 
  (Secondary) 
  rocks 
  

   also 
  occur. 
  My 
  friend 
  Prof. 
  Guiscardi, 
  of 
  the 
  University 
  of 
  Naples, 
  was, 
  in 
  

   1856, 
  able 
  to 
  give 
  a 
  list 
  of 
  no 
  less 
  than 
  112 
  species 
  of 
  fossils 
  derived 
  from 
  the 
  

   blocks 
  thus 
  ejected 
  from 
  the 
  crater 
  of 
  Vesuvius 
  (see 
  his 
  "Fossile 
  Fauna 
  des 
  

   Vesuvs" 
  in 
  Koth 
  s 
  'DerVesuv 
  und 
  die 
  Umgebung 
  von 
  Neapel,' 
  1857); 
  and 
  this 
  

   list 
  he 
  has 
  since 
  increased. 
  Near 
  the 
  great 
  crater 
  now 
  occupied 
  by 
  the 
  Lago 
  

   Bracciano, 
  I 
  found 
  great 
  masses 
  of 
  volcanic 
  agglomerate 
  containing 
  such 
  enor- 
  

   mous 
  quantities 
  of 
  angular 
  fragments 
  of 
  Subapennine 
  limestone 
  as 
  to 
  constitute 
  

   "volcanic 
  breccias 
  " 
  greatly 
  resembling 
  those 
  of 
  Mull 
  which 
  are 
  referred 
  to 
  in 
  

   the 
  text. 
  

  

  